1. Field of the Invention
The present invention relates to a stepping motor and a method of manufacturing the same.
2. Description of the Related Art
JP-2002-340631-A discloses a stepping motor that includes a casing having two casing members attached to each other, and a rotor and a stator disposed in the casing.
The rotor has a through hole, and a supporting shaft fixed to the two casing members is inserted in the through hole to rotatably hold the rotor. There is a clearance between the through hole and the supporting shaft so that the rotor rotates smoothly. Thus, the rotor is movable in a radial direction of the rotor for a distance of the clearance. When the rotor moves in the radial direction, the rotor may collide against the supporting shaft, and may generate a noise. Thus, the clearance between the through hole and the supporting shaft is filled with a lubricant so that rotor is restricted from moving in the radial direction and the noise generated by a collision between the through hole and the supporting shaft is reduced.
In the above-described stepping motor, the stator has a plurality of coils. Each of the coils generates a magnetic field by being supplied with electricity, and direction of magnetic fluxes of the magnetic fields cross each other. Thus, when electricity supply to each of the coils is switched in order and a direction of a magnetic flux passing through the yoke and the rotor is then switched in order, the rotor rotates on the supporting shaft. At the same time, the rotor moves in the radial direction of the rotor. When the rotor moves in the radial direction, the lubricant filling the clearance between the through hole and the supporting shaft may be flow out from the through hole. In a case where the lubricant flowing out from the though hole attaches at a portion in the vicinity of the through hole, the lubricant may be drawn in the through hole by the radial movement of the rotor. However, in a case where the lubricant flows out to another portion far from the through hole, the lubricant may be kept at the portion and may not return to the through hole. In the present case, the lubricant filling the through hole may be reduced as an operation time passes, and eventually, the rotor may directly collide with the supporting shaft and may generate a noise.